X-rays are used extensively in medicine, for both diagnostic and interventional purposes. Since x-ray radiation is potentially harmful to human tissue, it is necessary to accurately measure the amount of radiation to which a person is exposed. Too much radiation can permanently damage human tissue, while too little radiation may not allow the proper diagnosis and treatment of injury or disease.
X-rays interact with matter such as to produce secondary electrons, which are free electrons with large kinetic energy. The secondary electrons then create many more ions (electrically charged particles) as they travel and give up their energy. The ratio of the ionizations created by a secondary electron to a single ionization created by an x-ray photon is about 10,000 to 1. This is why x-rays are called indirectly ionizing radiation. Exposure is a measure of the ability of the x-ray radiation to ionize air. A commonly used unit of x-ray exposure is the roentgen. One roentgen is defined as the amount of radiation that will produce a charge of 3.336E-10 Coulombs in 1 cubic centimeter of dry air at 0.degree. C. and an atmospheric pressure of 760 millimeters of mercury.
It is known in the art that certain ion chambers, known as free air ion chambers, are used by standards laboratories and the like for measuring exposure. However, since free air ion chambers are bulky and relatively immobile, they are not suited for measuring x-ray generators in different locations. Instead, a compact and portable practical ion chamber is needed. Although these practical ion chambers come in a variety of shapes and sizes, the general concept is that a volume of air is contained within a cavity of solid material. A common variety of practical ion chambers is a flat parallel plate ion chamber. In this design, the charged plates which collect ions remain parallel to each other, as in a free air ion chamber. However, the plates are placed fairly close together, and the air volume between the plates in enclosed by solid walls.
For all of these enclosed volume chambers, the x-ray photons must first pass through a solid material (not air) before reaching the active air volume inside. The piece of solid material that the x-ray photons pass through is often called the entrance window of the chamber. As x-ray photons pass through this solid entrance window material, they produce secondary electrons. Many of these secondary electrons will travel into the active air volume. These electrons will, in turn, produce many more ions, which will be collected by the charged plates of the ion chamber. Since the definition of x-ray exposure involved ionizations produced when x-ray photons passed only through air (and ionization parameters are dependent on the material in which they occur), a direct measure of exposure cannot be determined using this type of chamber. Instead, this chamber must be calibrated by directly or indirectly comparing it to a free air ionization chamber. The practical and free air ion chambers are placed in the same x-ray field. The calibration factor for the practical ion chamber is then calculated by dividing the exposure measured with the free air chamber by the charge collected in the practical ion chamber. This practical ion chamber may then be used to measure the exposure of other x-ray beams by multiplying the charge collected in the ion chamber by the calibration factor.
For general medical x-ray procedures, there are two basic categories of x-ray beam qualities that must be measured by an ion chamber. These two categories are generally called diagnostic and mammography beam qualities. In mammography procedures, the kVp range of interest is about 20 to 50 kVp with very little added external filtration. Diagnostic procedures are normally defined from about 50 to 150 kVp with significantly more filtration in the beam. In addition, the anode material used to generate the x-ray spectrum is different (tungsten for diagnostic and usually molybdenum for mammography) which also causes differences in the x-ray spectra, and therefore the response of the ion chamber.
The entrance window must be thick enough to achieve electronic equilibrium, but thin enough so that attenuation is not a significant factor. Since the required window thickness is energy dependent, the desired thickness of an entrance window for mammography measurements may be different than that for diagnostic measurements. In the prior art, ion chambers only have one unique entrance window. Therefore, a choice must be made whether to optimize the entrance window thickness for mammography or diagnostic response.
Additionally, an entrance window that does not have an effective atomic number equivalent to that of air will result in an energy dependent ion chamber response. By adjusting the effective atomic number (a small thin piece of Al (atomic number=13) can be attached to the inside of the entrance window to increase the effective atomic number), an ion chamber energy response can be improved. However, one will likely never achieve a completely air equivalent window using the mixture of different materials (Plastic window, carbon coating, Al foil) that generally comprise entrance windows. Therefore, one must again choose an energy range over which to optimize the energy response. The amount of Al that must be added to optimize the diagnostic energy response is not necessarily the amount needed to optimize the energy response over the mammography energy range.
As a result, in the current state of the art of ion chambers, an ion chamber with an entrance window optimized for mammography energy response and with an effective atomic number optimized for mammography response has a relatively poor diagnostic response. Conversely, an ion chamber optimized for diagnostic response has relatively poor mammography response.
One example of an ion chamber that has been optimized for mammography energy response, as discussed above, is presented in the U.S. Patent to T. W. Slowey U.S. Pat. No. 5,115,134. As in the discussion presented above, this ion chamber has a single entrance window optimized for mammography response. This ion chamber provides a relatively flat energy response to x-rays over the mammography x-ray energy range of from approximately 20 kVp to 50 kVp. The U.S. Patent to H. Vlasbloem et al. U.S. Pat. No. 4,896,041 also discloses an ion chamber for medical use. This ion chamber employs two entrance windows which are of identical construction. That is, each window is optimized for the same x-ray energy range. No indication is presented that the windows might be constructed differently for use with different energy ranges.
Ion chamber manufacturers have solved this problem by selling two ion chambers, one optimized for diagnostic measurements and one optimized for mammography measurements. The mammography chamber will often have a much thinner window than the diagnostic chamber. Other manufacturers simply optimize the chamber for one of the two ranges, and accept a less than optimal energy response in the other range. This can result in significant inaccuracies for all measurements made in the poor energy response range.